This invention relates to a three-dimensional image shading method for displaying a three-dimensional image shaded by projecting a slice-stacked three-dimensional image consisting of a plurality of stacked two-dimensional images, or slices on a two-dimensional view plane (or projection plane), and particularly to one in which even though the gradients of pixel values of a part of an image are zero, that part is shaded according to the viewing angle by use of the volume rendering technique as long as it actually includes a tissue.
For the mutual connection among a plurality of CT images and the cubic effect, the plurality of CT images are sometimes stacked to form a three-dimensional image. The result is that the three-dimensional image is displayed in a simulative manner on the view plane. In this case, the volume rendering technique can be used to project the slice-stacked three-dimensional image on the view plane. This technique is described in detail in "Computer Graphics, Vol.22, No.4, Aug. 1988, pp.65-74". In the volume rendering process, incident light to the view plane is assumed and a reflection coefficient and a transmission factor are arbitrarily set according to the CT values. The amount of light incident to the view plane can be obtained as the product of the amount of reflected light from each voxel and the gradient of the contour plane of CT values. Thus where the gradient of the contour plane of CT values is zero, the amount of reflected light to the view plane is zero. The tissue corresponding to the zero-gradient of CT values is regarded as a hollow and displayed in the same black color as air.
It is assumed that a threshold level for gradient is set for a series of CT gradients as for example shown in FIG. 1 in order for a cross section of a head to be displayed. The gradient of CT values is very small because the CT values for the inside of the bone do not increase or decrease, and thus the pixel values on the view plane are also decreased by a shading process. Therefore, the inside of the bone is displayed in the same black color as air as if it were a hollow (see FIG. 2). The tissue which should be actually seen seems to be a hollow and may be decided to be a hollow by mistake.
The phenomenon of such erroneous hollow representation always occurs when thick tissues are attempted to be displayed by the volume rendering method and thus this problem must be solved.